The present disclosure relates to rooftop mount-type photovoltaic arrays. More particularly, it relates to a minimally penetrating support system for installing an array of photovoltaic devices to a rooftop structure, such as a lightweight commercial building.
Solar power has long been viewed as an important alternative energy source. To this end, substantial efforts and investments have been made to develop and improve upon solar energy collection technology, and in particular photovoltaic technology. As the cost of solar cells declines, the non-solar cell components necessary for maintaining a photovoltaic system relative to an installation site begin to dominate the overall system cost. Of particular interest are industrial- or commercial-type applications in which significant amounts of solar energy can be collected, but the corresponding installation/support system costs may negatively impact a potential user's purchasing decision.
Photovoltaic cells are the base technology associated with photovoltaic solar energy collection. Conventionally, a series or array of photovoltaic cells are formed on a single panel or laminate. The resultant photovoltaic laminate can subsequently be assembled to a frame (and other components) to create a photovoltaic module. Alternatively, two or more photovoltaic laminates can be assembled to a common frame. Regardless of whether photovoltaic laminates or photovoltaic module are supplied (collectively referred to as “photovoltaic devices”), most photovoltaic applications entail placing an array of photovoltaic devices at the installation site in a location where sunlight is readily present. This is especially true for commercial or industrial applications in which a relatively large number of photovoltaic devices are desirable for generating substantial amounts of energy, with the rooftop of a commercial building providing a convenient surface at which the photovoltaic devices can be placed. As a point of reference, many commercial buildings have large, flat roofs that are inherently conducive to placement of a photovoltaic array, and are the most efficient use of existing space. While rooftop installation is thus highly viable, the mounting systems employed to maintain the photovoltaic devices must address various constraints. For example, the installed photovoltaic array will be subjected to upward forces (e.g., wind) and downward forces (e.g., mass of the array, snow, etc.); the mounting system must maintain constant connection with the rooftop structure as well as long-term structural integrity under these conditions.
There are two main types of photovoltaic mounting systems used with flat commercial rooftops. Standard rack systems utilize heavy, elongated rails that are anchored to the rooftop by multiple roof penetrations (e.g., bolts driven through or penetrating the rooftop structure). These systems are typically engineered to withstand wind load calculations using standard methods. While available photovoltaic rooftop rack systems are capable of robustly maintaining and supporting numerous photovoltaic devices, the rooftop penetrations are undesirable (e.g., create opportunities for water leakage and otherwise permanently alter the rooftop integrity) and can be quite heavy. Thus, for lightweight buildings, conventional rack systems may not be useable. Conversely, lightweight, non-penetrating rooftop mounting systems have been developed that rely on pressure equalization to avoid overt loads imposed by pressure fluctuations above the building envelope. These non-penetrating systems are typically designed using wind tunnel methods to ensure that high wind speeds do not cause failures.
While the lightweight, non-penetrating photovoltaic array mounting systems enable a significant reduction in the amount of material and labor for installation, as well as very few or no penetrations into the roof envelope, in certain applications, concerns remain. In particular, many light commercial buildings incorporate a rooftop structure having a membrane mechanically attached to an underlying rooftop deck. The membrane is connected to the rooftop deck at limited, spaced-apart pinning points. Pressure fluctuations above the roof will cause the unattached sections of the membrane to flutter or inflate forcefully. Under these circumstances, the billowing membrane can present problems for non-penetrating photovoltaic array mounting systems in that the mounting system connection points, otherwise resting on a billowing membrane, will overtly flex, leading to possible failures. While retrofitted batten bars can be employed to hold the membrane to the rooftop deck at regular intervals, these batten bars are expensive and labor-intensive to install and require many rooftop penetrations, thereby partially or totally defeating the advantages of a “non-penetrating” mounting system.
In light of the above, a need exists for a minimally penetrating photovoltaic rooftop support system that maintains structural integrity in the presence of various forces, including a billowing membrane.